Cost-effective recycling of materials, such as glass, has become an increasingly important issue because of stresses on the environment and scarcity of resources. Concern about these issues has prompted governmental involvement, including the establishment of governmental guidelines, e.g., “bottle bills.” Because of governmental requirements and environmental awareness, American consumers are now recycling at a greater rate than ever before.
Increased recycling of materials reduces the amount of materials, such as glass, plastics, paper, etc., that enter land fills or other waste disposal points. Additionally, recycling significantly reduces the need for manufacturers to use “virgin” materials, and thus preserves environmental resources. Further, the use of recyclables in place of virgin raw materials often reduces energy requirements, eliminates process steps, and reduces waste streams, such as air emissions during product manufacturing. For example, recycled glass requires less energy and emits fewer contaminants during the glass manufacturing process than virgin raw materials. Many states have placed demands on glass manufacturers that require new glass bottles to contain a minimum percentage of recycled glass. For example, in Oregon, glass container manufacturers are required to use at least 35% post-consumer cullet, which is broken pieces of glass. California has even more aggressive laws requiring glass manufacturers to increase use of recycled content to 65%.
However, there are associated issues of complying with these governmental mandates. Prohibitive sorting costs have made it difficult for suppliers to process an adequate quantity of single-colored recycled glass. Often, the glass coming into a material recovery facility (MRF) and/or a glass processing facility, e.g., a site where cullet is cleaned and prepared for shipment to glass manufacturers, is broken, contaminated with other materials, and of mixed color. Such material was previously unusable for glass manufacturing and was used in low value applications or simply placed in a landfill. Currently, however, regulatory requirements and other issues are forcing glass manufacturers to increase the amount of mixed cullet used in their furnaces which, in turn, has established the need to use mixed cullet in the manufacture of glass.
Generally, fewer problems exist when recycling clean sources of glass cullet that are of the same composition as products being manufactured. However, since most cullet is derived from consumer waste, the main issue is how well-segregated the different glass waste streams are, and the consequent level of contamination. Foreign material such as ceramics (such as pottery and china, from restaurants and/or bars); stones, gravel and/or dirt (from poor storage of cullet); ferrous metals (from bottle tops and other scrap); non-ferrous metals (lead foils from wine bottles or aluminum bottle caps); and organics (labels and excessive food residue) can all render a batch of mixed cullet unusable.
In the glass-making process, non-ferrous metals can give rise to higher emissions or can settle out as a layer at the base of the glass furnace. Ceramics may not be incorporated into the melt, and although organics will burn off in most furnaces, they create offensive odor and may present a problem in the crushing machinery used to produce the cullet. Thus, cullet quality and the levels of contamination are of major concern. It would be beneficial to develop a process for reusing mixed cullet, wherein mixed cullet is used like color-sorted cullet, to make new and useful glass products.
Glass manufacturers are especially concerned about the quality of glass cullet supplied from post-consumer streams. With glass batch formulations now able to contain increased amounts of mixed cullet (e.g., glass of mixed-colors, typically green, amber, and flint/clear), the mixed cullet or single-colored cullet glass received by glass manufacturers should consistently meet the specifications given by the suppliers. Glass manufacturers rely upon these specifications, which specify glass compositions and levels of impurities, to develop a glass batch recipe, or formulation, for manufacturing bottles.
Generally, the glass making operation is a semi-continuous batch process where each batch, containing cullet and other raw materials, is heated to product a glass melt having a desired final composition. The batch raw materials may also contain, in whole or part, virgin raw materials that were not derived from post-consumer waste streams. The desired final composition is established by a batch formulation using data concerning the number, weight, and/or and character of the cullet and virgin raw materials. Individual batches, however, may be greatly affected by the amount and character of cullet additions. For example, cullet additions may affect, amongst other processing variables, the viscosity and absorptivity of the molten glass mixture and the color of the final product. Thus, batch-to-batch variability may impact end product color, composition, and quality. The cullet specifications provided by a material recovery facility, or other glass processing facility, may not be accurate and glass manufactures must often guesstimate batch formulations due to overly generalized, or simply inaccurate, data. For example, supposed single-color amber cullet from a processor may actually contain five to ten percent of other colored glass and/or varying levels of ceramic and organic contamination. This discrepancy between the processor's cullet specification and the actual composition/character of the supplied cullet will likely result in inter-batch variability and negatively impact glass product color uniformity and quality. Cullet composition variability has been tolerated by the glass industry so long as use of mixed cullet was low, thus not significantly affecting the glass product color or composition. As mixed cullet use levels increase, there is an greater need to reduce or mitigate color variability to ensure that the color of glass manufactured from the mixed cullet is of uniform color, composition, and quality. Thus, real-time analysis of mixed cullet composition coupled with real-time batch formulation, as the cullet is introduced into the glass manufacturing process, would be beneficial in ensuring uniformity of as-manufactured glass products.
One method of using mixed-colored cullet for glass manufacturing is described in U.S. Pat. No. 5,718,737, entitled, “Method of Recycling Mixed-Colored Cullet into Amber, Green, or Flint Glass.” The '737 patent, herein incorporated by reference, describes how mixed-colored cullet glass is recycled into amber colored glass by regulating the additive amounts of iron, carbon, sulfur, and sulfur compounds in the mixture to impart the desired reddish-brown hue. While the '737 patent provides a suitable method of using mixed-colored cullet for glass manufacturing, it does not address the variability within the batch formulations as a result of the variations in the mixed cullet supply. Nor does the '737 patent provide real-time analysis of the composition of mixed cullet as it is introduced into the glass manufacturing process to allow for batch formulation adjustment to ensure end-product uniformity.
U.S. Pat. No. 6,230,521, entitled, “Method of Recycling Batches of Mixed-Color Cullet into Amber, Green, or Flint Glass with Selected Properties,” herein incorporated by reference, describes an automated method for recycling mixed-colored cullet glass into new glass products. A computer-controlled process identifies the virgin glass raw materials, the desired target glass properties, the composition of a batch of mixed-colored cullet, and the quantity of cullet to be used in the glass melt. The computer controlled process automatically determines the proper amounts of virgin glass raw materials to add to the batch of mixed-colored cullet so that recycled glass is produced that has the desired coloring oxides, redox agents, and glass structural oxides in the proper proportion. While the '521 patent provides a suitable method of recycling mixed-color cullet glass into new glass products, it does not provide a method of removing impurities from the cullet within the glass manufacturing process prior to being introduced within the glass batch. One method of identifying cullet composition is provided by U.S. patent application Ser. No. 10/988,018, entitled “Method of Analyzing Mixed-Color Cullet to Facilitate Its Use in Glass Manufacture,” herein incorporated by reference.
Mixed cullet is processed and cleaned by entities, such as an material recovery facility and/or a cullet processor, before it is shipped to a glass manufacturer. The composition of mixed cullet depends on municipal sources and the color balance of the cullet may vary over time in a way that may not be reflected in a supplier specifications when the glass cullet is shipped to the glass manufacturer. These variations in the color of the mixed cullet should, ideally, be mitigated by the glass batch formulation in order to maintain uniform end-product color. Furthermore, the supply of mixed cullet often contains residual contaminants, such as ceramics and other impurities, that have escaped the cleaning process. These impurities, also of serious concern to glass manufacturers, will likely contribute to inconsistent quality and composition of the as-manufactured glass if modifications to the batch formulation are not made.
There exists a need in the art for a method of mitigating compositional variations in glass batch formulations due to the mixed cullet variations. In order to further optimize the cullet and therefore glass batch quality, there also exists a need in the art for a system and method for further removing impurities from the cullet within the glass manufacturing process prior to the cullet being introduced within the glass batch melt.